1. Field of the Invention
This invention relates to load cells, more particularly to multiaxis load cells capable of measuring forces and moments about three orthogonal axes.
2. Description of the Prior Art
Multi-axis load cells have long been available for various purposes, typical examples being found in U.S. Pat. Nos. 3,640,130; 3,433,064; 3,600,942; 3,492,864; 3,513,457; 3,513,431; 3,693,425; 3,552,199; 3,427,875; and 3,618,376. The use of multi-axis load cells has been heretofore been proposed in the context of automated assembly operations in which a manipulator device is controlled so as to perform certain assembly tasks by means of so-called "force feedback steering" in which the reactive forces transmitted back through the manipulator as the assembly task is carried out are used to generate control signals to perform the assembly task. Such an application requires a load cell having simple, lightweight physical structure and in addition must be capable of functioning over a wide range of dynamic load levels since very lightweight parts, i.e., washers, pins, etc., and relatively heavy objects, i.e., castings, etc., and may be expected to be handled by the manipulator, and the reactive loads may also vary widely depending on the dynamic nature of the specific assembly task being performed. Studies to date have also indicated that for such force feedback steering to be effective force levels along each of the orthogonal axes and bending moments about each of these axes must each be measured quite accurately. An approach to multiaxis load cells suitable to this application has been described in a published Master's Thesis entitled "Force Feedback Steering of a Tele-Operator System" by R. C. Groome, Jr., MIT Draper Laboratory Report T-575, August, 1972. This arrangement consists of a hub connected to a housing by means of four cantilever bars arranged in a cross pattern, each of the bars being fixed to the housing while being supported so as to be slidable along its axis with respect to the housing. Resistive strain gages are applied to each of these bars so as to measure the lateral forces experienced by each of the cantilever bars. As described therein, the forces so sensed may be resolved to obtain the three force and three torque components experienced by the load cell.
While this approach is extemely simple and effective, the use of resistive foil strain gage elements on the cantilever bars to measure the forces limits the dynamic range of load levels which are capable of being measured by the assembly since the resolution of strain gages of this type is quite limited.
Therefore, it is an object of the present invention to provide a load cell which is both simple and is capable of sensitively measuring multi-axis loads along a wide dynamic range of load levels.